The invention relates to control systems used for stabilizing a vehicle, such as wheel slip control systems, antilock systems, braking assistants or vehicle dynamics control systems.
The triggering of an automatic control system for stabilizing a vehicle, for example, a wheel slip control system, typically does not take place before the start of any instability of a vehicle, for example, when a permissible wheel slip limit is exceeded; i.e. when the wheel slip is outside the limits permissible with respect to vehicle stability.
The reaction of the automatic control system takes place within a different time period as a function of vehicle-specific integration environments (components, hardware, onboard wiring system, etc.). Under unfavorable marginal conditions, the reaction may not take place before a point-in-time at which the control deviation is already quite advanced which, in turn, results in intensive interventions of the control system.
The high intensity of the control interventions leads to an increased load profile of the onboard wiring system and of components as well as to loss of comfort in the driver's perception.
It is therefore an object of the invention to provide a process and a corresponding system which decrease the high intensity of the control interventions of such automatic control systems and reduce the disadvantages connected therewith.
This and other objects are achieved by a process and corresponding system for limiting a torque or an amount characteristic thereof of a control loop used for stabilizing a vehicle. The process detects a coefficient of friction (μs,r, μs,f), determines a limit value (Mg) for the torque or the amount characteristic thereof as a function of the estimated coefficient of friction (μs,r, μs,f), and limits the torque or the amount characteristic thereof to the limit value (Mg).
A first aspect of the invention is aimed at processes for limiting a torque, or an amount characteristic thereof, of a control loop used for stabilizing a vehicle. The control loop may, for example, include a wheel slip control system, an antilock system, a brake assistant system or a system controlling the dynamics of vehicle movements. The torque preferably causes a buildup of force in the longitudinal direction of one or more wheels, because typically no active torque influence takes place in the transverse direction. The torque may, for example, be an acceleration torque or a deceleration torque. The resulting longitudinal-force buildup of the vehicle takes place, for example, in the deceleration or acceleration direction of the vehicle and can influence the dynamic longitudinal and/or dynamic lateral behavior.
An amount characteristic of a torque is an amount or parameter that has a certain connection with a torque, particularly a linear connection. The value may, for example, be power, force or acceleration information. Thus, in the application, all information concerning the torque also applies to such a characteristic value in the same fashion. And, as used herein, the term “torque” includes an amount characteristic thereof.
In the process, a coefficient of friction is determined. For detecting the coefficient of friction, an additional sensor system for determining the coefficient of friction can be used. The coefficient of friction is preferably estimated by way of vehicle-internal quantities. The estimation of a coefficient of friction by way of vehicle-internal quantities is described in Chapter 2.3.1 of the reference book “Verbesserungspotenzial von Stabilisierungssystem im Pkw durch eine Reibwertsensorik” (“Improvement Potential of a Stabilization System in a Passenger Car by Means of a System for Sensing the Coefficient of Friction”), by Ingo Weber, Fortschr.-Ber. VDI Reihe 12, No. 592, VDI Verlage 2005. The statements concerning the estimation of a coefficient of friction made there are hereby expressly incorporated by reference herein. The estimation of the coefficient of friction by means of vehicle-internal quantities can be supported by additional sensor systems (for example, systems integrated in the tire, or camera-based). The estimated coefficient of friction can thereby be checked with respect to plausibility by use of a sensor in the tire or by use of the camera image. The coefficient of friction may, for example, be a coefficient of friction of a tire, of a vehicle axle or a coefficient of friction of the entire vehicle.
A limit value for the torque or the amount characteristic thereof is determined as a function of the coefficient of friction. This limit value may, for example, be oriented according to the torque radius of a Kamm's friction circle, which is a function of the coefficient of friction. In particular, it may be slightly larger than this torque radius. The torque or the amount characteristic thereof is limited to the thus determined limit value.
The limitation by the limit value can take place, for example, in the upward or downward direction. A limitation of the drag torque by the limit value in the downward direction makes sense, for example, in the case of an automatic engine drag torque control, where the negative torque is briefly increased in the acceleration direction, and the drag torque is thereby reduced in order to keep the vehicle stable. The method of limiting the drag torque can also be used in a drive drag torque control of a hybrid vehicle with recuperation, where the recuperation torque of the electric machine is controlled in the coasting operation, instead of being used in an engine drag torque control.
As a result of the determination of a limit value for the control disposed on the input side of the current control, the torque (or the amount characteristic thereof) can be limited to a value adequate for the driving situation, so that the intervention of the control system in the vehicle driving performance is clearly reduced. This leads to a reduced load profile of the on-board wiring system and components as well as to the improvement of comfort in the driver's perception.
The process according to the invention is therefore to some extent a pilot control disposed on the input side of the control loop or a preconditioning of the field. However, in contrast to a classic pilot control, this pilot control determines a limit value within the control loop, for example, for a control variable of the control loop or for a value within the controlled system (particularly within the driving or braking control), instead of directly applying a value to the control variable of the control loop. The process according to the invention preferably determines a limit value for the transmission behavior (and optionally for the transmission dynamics) within the controlled system. The pilot control preferably limits the occurring control difference, for example, in the absolute value or in the time behavior, by influencing the controlled system or the transmission behavior of the controlled system essentially independently of the control loop; the control on the output side is thereby weakened or the initial control threshold is not reached.
The coefficient of friction is preferably determined when the reaching of a stability limit is detected or in the case of a control activity (which typically takes place when the stability limit has been reached). The reaching of a stability limit or control activity can be recognized by analyzing a slip signal or by analyzing a yaw rate signal. For example, it can be determined that a slip exceeds a certain limit (a certain overslip is therefore present), and/or a yaw rate exceeds a certain limit (for example, when the yaw rate exceeds a so-called Ackermann yaw rate in the downward or upward direction). For this purpose also, an arbitrary control signal (such as a binary digital signal) can be analyzed, which indicates a control activity.
When the stability limit, i.e. the traction limit has been reached, conclusions on the coefficient of friction can be drawn particularly well, because Kamm's static circle is then intersected and the maximal static friction force is reached.
According to a preferred embodiment, the limit value will then be selected such that the control system can pass through the detected stability limit. This permits an activation of the control (which is typically activated at the stability limit) despite the limitation to the limit value. If the amount of the limit value were selected such that there could no longer be a passing through the detected stability limit, the control system could later no longer be activated.
Preferably, when the reaching of a stability limit or the activity of the control loop is detected again, the coefficient of friction will be determined again. Among other things, this makes it possible to adapt the determined coefficient of friction in the downward direction when the nature of the road changes, for example. Specifically, when the traction limit has been reached again, the current coefficient of friction may have decreased and the determined coefficient of friction can then be reduced correspondingly.
The determined coefficient of friction is preferably stored. As long as no adaptation of the determined coefficient of friction takes place, the process can take place by using the stored coefficient of friction. The torque limitation at the current point-in-time will then not take place as a function of the current actual coefficient of friction but rather as a function of a coefficient of friction that was determined from vehicle-internal quantities which describe a condition of the past.
According to an advantageous embodiment, the coefficient of friction is therefore stored. In addition, a current coefficient of friction is determined continuously. The determined coefficient of friction is updated as a function of the stored coefficient of friction and of the current coefficient of friction. When the reaching of a stability limit is detected or an activity of the control system takes place, the updated coefficient of friction will preferably correspond to the current coefficient of friction. The determined coefficient of friction can thereby be reduced (in this manner, the process can “learn down” to a lower value). If the traction limit was not reached or no activity of the control system takes place, the updated coefficient of friction will correspond to the maximum from the current coefficient of friction and the stored coefficient of friction. When the currently determined coefficient of friction therefore is above the stored coefficient of friction, the coefficient of friction can thus be increased (the process can therefore “learn up” to a higher coefficient of friction).
In the process, the coefficient of friction is preferably determined as a function of a torque or of an amount characteristic thereof, particularly a torque or a characteristic value when reaching a traction limit or a control activity. This may, for example, be a wheel torque.
As indicated above, when determining the limit value, it should preferably be taken into account that the control loop will still be capable of passing through the previously detected stability limit. For this purpose, the amount of the limit value per wheel should preferably be larger than the amount of torque per wheel that is used for determining the coefficient of friction.
According to a preferred embodiment, for determining the limit value, a maximal torque or an amount characteristic thereof is determined first as a function of the determined coefficient of friction. The limit value is determined as a function of this maximal amount, in which case the limit value will exceed the maximal amount by a certain extent, for example, by 5% to 15%, particularly by 10%. However, in this case, the limit value should preferably be at least 150-750 Nm (wheel-related axle torque, i.e. behind the differential), for example, 250 Nm or 500 Nm above the maximal amount. Depending on the vehicle and control system, the limit value is applied, for example, by 10% but at least by 250 Nm wheel-related axle torque above the maximal amount.
The maximal amount preferably corresponds to the maximal amount of a torque that causes a buildup of force in the longitudinal direction of one or more wheels thus of a wheel torque or of the sum of several wheel torques in the circumferential direction. Here, the maximal amount of the torque is estimated as a function of the determined coefficient of friction and as a function of a lateral acceleration. This will be explained below in greater detail by means of an embodiment.
According to a preferred embodiment of the process, a vertical wheel force or an amount characteristic thereof is also determined as a function of the vertical wheel force or of the amount characteristic thereof.
A further aspect of the invention is aimed at an arrangement for determining a limit value for a torque (or an amount characteristic thereof) of a control loop used for stabilizing a vehicle.
The arrangement includes devices for detecting a coefficient of friction. In addition, devices are provided for determining a limit value for the torque (or the amount characteristic thereof) as a function of the determined coefficient of friction. These devices operate as described above.
The above statements concerning the process of the invention according to the first aspect of the invention correspondingly apply also to the arrangement according to the invention.
The invention is also aimed at an automatic control system, which includes the above-mentioned arrangement for determining a limit value. The control system may provide devices (for example, a limiting device) which are used for the limitation to the limit value.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.